Storing ArrayObject as WordPress transient

collection.php

<?php
/** ..... */
public function all() {
  if ( false === ( $collection = get_transient( '_optin_cache_' . $this->user->ID . '_all' ) ) ) {
  	$optins = get_posts(
  		array(
  			'post_type'     => 'optin_saas',
  			'author'        => $this->user->ID,
  			'no_found_rows' => true,
  			'cache_results' => false,
  			'nopaging'      => true
  		)
  	);
  	if ( empty( $optins ) ) {
  		throw new \Exception( 'Optin forms were not found for this account' );
  	}
  
    // Create the collection object. Extends ArrayObject.
  	$collection = new OptinCollection();
  
  	// Create the optin object and add to the collection
  	foreach ( $optins as $optin ) {
  		$o = new Optin( $this->user );
  		$collection->append( $o->build_from_wp_post( $optin ) );
  	}
  
    // Save the collection as a transient
  	set_transient( '_optin_cache_' . $this->user->ID . '_all', $collection, DAY_IN_SECONDS );
  
  }

  return $collection;
  
}
/** ....... */